In optical waveguides used in optical data transmission and optical laser cavities, light propagates in one spatial direction. These waveguides use total internal reflection at an interface between two media with relatively higher and lower refractive indices to direct the light. Total internal reflection causes the light to propagate in the medium with the higher refractive index.
Periodic dielectric structures can also be used to direct light propagation. In periodic dielectric structures, light propagation is analogous to electron propagation in a crystal. If the wavelength of the light is of the order of the dimensions of the lattice, a photonic bandgap (PBG) forms. The PBG is a wavelength range in which photons cannot propagate through a periodic dielectric structure. If incident light's wavelength is in the PBG, the incident light is reflected off the periodic dielectric structure rather than transmitted through the structure. Period dielectric structures whose lattice lengths are of the order of wavelengths of near infrared or visible light are often referred to as PBG structures. Light with a wavelength in a PBG can propagate down a narrow channel in a PBG structure.
Proposals exist for using PBG structures to make optical cavities. An article entitled “Two-Dimensional Photonic Band-Gap Defect Mode Laser” by O. Painter et al. appearing in the 11 Jun. 1999 issue of Science (p. 18 et seq.) describes the formation of a laser cavity in a two-dimensional (2D) a 2D PBG structure. The laser cavity is fabricated in a group III-V crystalline semiconductor and uses a channel in a 2D PBG structure and a defect to form the laser cavity.